FIG. 1 shows a block diagram of a power conversion system that can be used with a fuel cell stack 10. Since the fuel source normally cannot keep up with load transients, it is often necessary to include, in addition to a power conditioner 12A, a battery 14 or other energy storage device to provide power during load transients. FIG. 1 shows a switch S1 used to disconnect the fuel cell stack from the system. Switch S1 may also be used to prevent reverse flow of current into the stack. Power conditioner 12A processes the power from the fuel cell stack 10 to a common dc bus 16. Power conditioner 12A must operate over a large range of input voltage since the fuel cell stack voltage changes significantly with load. A second power conditioner 12B controls the flow of power between the battery 14 (or other storage element) and the dc bus 16. A load 18 and an auxiliary power requirement 20 are also shown in FIG. 1.
The fuel cell system balance of plant (BOP) requires auxiliary power 20 to operate. The BOP may comprise blowers, pumps, or sensors. The auxiliary power 20 would normally be drawn from a relatively constant voltage source to reduce the cost of the BOP components. It would therefore be standard practice to draw the auxiliary power either from the battery 14 (through the second power conditioner 12B) or from the common dc bus 16, as is shown in FIG. 1. Since the BOP components ultimately derive their power from the fuel cell stack, the system efficiency would be higher if the BOP components were connected to the common dc bus rather than to the battery. (I.e.—it saves the extra power loss that would occur in power conditioner 12B to connect the BOP components to the common dc bus rather than to the battery.)
FIG. 2 depicts a specific example of the system illustrated in FIG. 1. The system shown in FIG. 2 could be used as a telecommunications power supply. In this example, the load 18 comprises a 48V battery (not shown) in parallel with electronic loads, and so the output of the fuel cell system should be maintained at the float voltage of the battery, approximately 55V for a nominally 48V battery. In this specific example, a first power conditioner 12A′ could be a dc-to-dc converter (or boost converter when it is used to boost the fuel cell voltage) for providing a dc voltage to the dc bus. A second power converter 12B′ would have to allow the power to flow in both directions to permit the battery 14′ to provide power to the load 18 as well as to be recharged from the fuel cell stack 10′.
FIG. 2 also shows the maximum power-output or power-handling capability of each system component. The auxiliary load 20′ is assumed to be a maximum of 400 W and the telecommunications load 18 is assumed to be 1 kW maximum. The fuel cell stack 10′ should be able to provide steady-state power to both the telecommunications load 18 and to the auxiliary load 20′. The fuel cell stack 10′ should therefore have a maximum power output capability of at least 1.4 kW. If the fuel cell stack is taken off-line due to momentary fuel problems, etc., then the battery 14′ should be able to provide the entire load. The battery would therefore also be sized to provide 1.4 kW for the maximum expected period of fuel cell stack non-availability. Both of the power conditioners 12A′ and 12B′ should be sized to handle 1.4 kW of power. The system therefore contains 2.8 kW of power electronics (not including any power processing within the auxiliary or telecommunication loads).
FIG. 3 shows another possible topology to accomplish the same task as the system in FIG. 2. The system in FIG. 3 employs 2 diodes to share power between the battery 14′ and the fuel cell stack 10′. This topology may or may not reduce the cost of the power electronics depending on the required charging rate of the battery and the cost of the diodes. It also causes an unwanted side effect in that the diodes produce a significant loss of efficiency in the circuit.
A goal of the present invention is to provide an improved power conversion system that avoids the shortcomings of the approaches described above.